1. Field of the Invention
The invention concerns an instrument for measuring particle fallout on a surface using a telltale plate.
2. Related Art
In many industries it is necessary to protect all kinds of products, objects, devices, etc from contamination, in particular by particles. This is the case in particular in the fabrication of integrated circuits, measuring instrumentation for aircraft or spacecraft, medical instrumentation, thin film devices, pharmaceutical preparations, photographic emulsions, etc. In all such applications the slightest contamination can reduce output or shorten service life. Monitoring contaminants that can be deposited on a product is therefore vital for this type of industry.
For this reason, production and inspection are carried out in "white rooms" and the air entering such rooms is filtered to minimize the risk of contamination. Additionally, the pressure is raised relative to the external surroundings. However, because of the activity of the personnel present, a large quantity of particles is emitted into the environment of such rooms. The effect of gravity on such particles causes them to settle on all surfaces of the room and, more importantly, on the products being fabricated or assembled.
Initially the contamination of products in "white rooms" was not measured directly. Instead of such direct measurement the size and the number of particles in suspension per unit volume were measured. However, this method could give only a highly uncertain indication of the exact degree of contamination of the products themselves due to the fallout of particles. The larger particles generated by a given operative tend to settle faster in the immediate vicinity of the operative in question. What is more, she operative is likely to move around inside the protected enclosure. Th above method had major drawbacks.
It has been proposed to measure the contamination of surfaces due to the fallout of particles using sampling surfaces in the form of telltale plates. These telltale plates are placed in the immediate vicinity of the product during fabrication or the device during assembly. Particles in suspension can therefore settle freely on the telltale plates for a specified time period. At the end of this specified time period the aforementioned plates are collected and the particle deposit is measured using optical apparatus based on a photometer.
The above method has a number of advantages. Firstly, the accuracy of the measurements is greatly enhanced: in particular it allows for the heaviest particles produced by the operatives, since the plates are disposed near them when they are working on a given product or device. It is simple to use and less costly than methods previously used. It can also measure the quality of the renewed air, for which purpose it is sufficient to carry out measurements during periods of inactivity.
Apparatus for carrying out measurements by the above method has been developed by SAAB AKTIEBOLAG and is described in British patent application GB-A-1 145 657.
FIG. 1 is a diagrammatic illustration of the operating principle of measuring apparatus of the above kind.
The telltale plate P.sub.la is contained in a plate-carrier (not shown) to prevent accidental contamination. During the measurement it is placed on a mobile drawer T.sub.i (position I: out of measuring apparatus, position II: measuring position). Once introduced into the measuring apparatus PFO the telltale plate P.sub.la is illuminated at grazing incidence by two intense light beams f.sub.1 and f.sub.2. The two beams f.sub.1 and f.sub.2 are from a single light source S.sub.O of the halogen lamp type. A set of appropriately oriented mirrors M.sub.1 through M.sub.3 and of focusing lenses L.sub.1 -L.sub.2 and L.sub.3 -L.sub.4 splits the main beam f emitted by the single source S.sub.O into two sub-beams f.sub.1 and f.sub.2 and directs them onto the telltale plate P.sub.la at grazing incidence in two mutually orthogonal directions.
The particles deposited on the telltale plate P.sub.la diffuse the incident light in all directions. A cadmium sulfide photo-electric cell P.sub.HO measures the light diffused in a direction orthogonal to the surface of the telltale plate P.sub.la, i.e. at a diffusion angle of 90.degree.. The luminous intensity causes its internal resistance to vary.
To be able also to observe the light diffused along the measuring axis by means of a microscope M.sub.i, a mirror M.sub.MO is provided that is rotatable about an axis parallel to the surface of the telltale plate P.sub.la. An appropriate mechanism M.sub.e drives the mirror M.sub.MO. The mirror M.sub.MO is raised (position I) to enable observation by means of the microscope M.sub.i or at an angle of 45.degree. (position II) to the surface of the telltale plate P.sub.la and to the diffused beam L.sub.us to deflect the beam L.sub.us 90.degree. towards the photo-electric cell P.sub.HO. The output signals are transmitted to electronic circuits A.sub.MP which include a measuring amplifier and circuits controlling the light source S.sub.O.
The amplitude of the electrical signals at the output of the measurement amplifier is transmitted to measuring apparatus. They are representative of the quantity of particles deposited per unit surface area on the surface of the telltale plate P.sub.la. Knowing the surface area and the time of exposure of the latter, it is possible to deduce the degree of contamination of the objects near which it was placed. If a number of plates are used an average can be calculated to enhance the accuracy of the measurement.
The apparatus described has three measurement ranges, the measurement amplifier A.sub.MP being provided with electronic circuits for switching between them automatically. The amplifier is fed with a stabilized voltage so that the measurements are nor sensitive to the variations in the mains power supply or to temperature variations. The luminous flux from the halogen lamp S.sub.O is stabilized by means of an additional photodiode (not shown) in a feedback circuit.
A calibration circuit (not shown) is also necessary. A potentiometer (not shown) is used to adjust the measurements in the calibration phase.
The above type of measuring apparatus represented a real step forward compared to previously known measurement methods. Nevertheless, it is not completely free of drawbacks. The latter can be summarized as follows:
the usable measuring area is small: it is substantially limited to a 15 mm circle; PA1 aligning the incident light is difficult, in particular if the apparatus has been moved: these alignment problems are associated with the fixing of the halogen lamp and the four mirrors; PA1 the use of a white light source restricts the measurement to non-fluorescent particles; PA1 the optical background is typically limited to approximately 10 ppm on uncontaminated plates due to problems of stray light. PA1 The usable measuring area can be increased, typically from a 15 mm diameter circle to a 30 mm diameter circle, using a telltale plate with the same dimensions. This represents an enhancement factor of four, which is very important for statistical interpretation. PA1 It is no longer necessary to use mirrors because the annular structure of light-emitting diodes can be fixed directly to an annular frame around the telltale plate. The angle of incidence is fixed. It is then possible to determine once and for all an optimized structure to obtain a maximum of diffused light for the aforementioned area and the lowest possible acquisition of stray light. This structure avoids re-adjustment of the optical paths (beams f.sub.1 and f.sub.2 in FIG. 1). PA1 The light-emitting diodes generate monochromatic light, advantageously at a wavelength of 630 nm, which eliminates fluorescent effects when a 630 nm optical filter is placed in front of the electro-optic detector. PA1 The optical enclosure is painted black enabling optical acquisitions typically less than or equal to 1 ppm (parts per million) (which must be compared with the typical value of 10 ppm previously referred to). PA1 The structure of the apparatus in accordance with the invention authorizes very simple electronics associated with very low power consumption and consequently a very short warm up time. A very stable base is no longer necessary, because it is no longer necessary to adjust any mirrors, there being no mirrors, as previously indicated. Finally, because of the miniaturization that is possible, both mechanically and electronically, the apparatus has a much smaller overall size than the prior art apparatus shown in FIG. 1. PA1 As described in detail below, the overall structure of the measuring apparatus comprises two main compartments: an optical enclosure and a detector part. As in the prior art, this arrangement authorizes either the use of a detector, in the case of the invention advantageously a CCD (Charge-Coupled Device) type detector, or the use of a microscope. PA1 The complex internal calibration system is no longer necessary either. This eliminates further causes of errors in the measurements because of the complicated mechanism required to adjust the angle of deflection of the mirrors (reflecting the beams f.sub.1 f.sub.2) and a removable mirror (M.sub.MO)